Hydrological Applications of Remote Sensing and Remote Data Transmission (Proceedings of the Hamburg Symposium, August 1983). IAHS Publ. no. 145.

Collection of data using the METEOSAT DCP retransmission system

R, W, HERSCHY Department of the Environment, Romney House, 43 Marsham Street, London SW1P 3PY, UK

ABSTRACT There are only some 30 data c o l l e c t i o n p la t forms (DCPs) in Europe l inked to METEOSAT. One reason why use r s have not taken more advantage of t h i s f a c i l i t y l i e s in the method t h a t the data are r e tu rned from Darmstadt by l a n d - l i n e . Unt i l now t h i s r e t u r n of da ta was v i a the Global Telecommunications System and the paper b r i e f l y de sc r ibe s t h i s method. The new method of d i s t r i b u t i o n of da ta by the European Space Agency (ESA) c o n s i s t s of r e t r a n s m i t t i n g t he DCP da ta v ia the s a t e l l i t e t o small r e c e i v i n g s t a t i o n s i n s t a l l e d by the u s e r . These s t a t i o n s have a r e ce iv ing d i sh of about 1.5m diameter . Using spare capac i ty on the Weather Facsimile System (Wefax), the u s e r ' s da ta are r ece ived d i r e c t l y wi th in about 4 min of t r a n s m i s s i o n . The a l e r t f a c i l i t y w i l l remain. A b r i e f d i scuss ion of ESA's p r o v i s i o n a l fu tu re s a t e l l i t e programme i s a l so p resen ted .

Collecte de données utilisant le système de retransmission DCP lié a METEOSAT RESUME I I n ' e x i s t e en Europe que quelque 30 p l a t e s -formes de r e c u e i l de données (DCI1) r e l i é e s a METEOSAT Une des r a i s o n s pour l a q u e l l e l e s u t i l i s a t e u r s n ' on t pas t i r é davantage p a r t i de ce système r é s i d e dans l a na tu r e de l a méthode u t i l i s é e pour t r a n s m e t t r e l e s données de Darmstadt par l i gnes t e r r e s t r e s . Jusqu ' à p r é s e n t , c e t t e r e t r ansmis s ion de données s ' e f f e c t u a i t par voie du Système de Télécommunications P l a n é t a i r e selon l a méthode brièvement d é c r i t e dans l e p résen t exposé. La nouve l le

' méthode de d i f fus ion de données par l 'Agence S p a t i a l e Européenne (ASE) c o n s i s t e a r e t r a n s m e t t r e l e s données DCP par voie du s a t e l l i t e ve r s des p e t i t e s s t a t i o n s mises en p lace par l ' u t i l i s a t e u r . Ces s t a t i o n s sont dotées d ' an tennes pa rabo l iques r é c e p t r i c e s d 'un diamètre d ' env i ron 1.5 m. Les données de l ' u t i l i s a t e u r , t r ansmises en u t i l i s a n t l e s c a p a c i t é s d i spon ib l e s du WEFAX (système de f ac s imi l e météoro log ique) , sont reçues dans un d é l a i d ' env i ron 4 min à p a r t i r de l eu r t r a n s m i s s i o n . Le système d ' a l e r t e s u b s i s t e r a . Un bref aperçu du fu tu r programme de s a t e l l i t e s p r o v i s o i r e de l'ASE e s t également p r é s e n t é .

INTRODUCTION

It has been shown that data collection platforms (DCPs) used in 109

110 R.W.Herschy

conjunction with geostationary satellites offer a reliable means of telemetering water management data (Herschy, 1982; Shope & Paulson, 1980). One of the problems, however, particularly in Europe, has been to find an acceptable cost effective method of receiving the data. A direct reception station which allows the user to receive DCP messages directly from the satellite is too expensive at this stage for most water management purposes. This type of station consists of a fixed-position dish antenna with a diameter of 4.5 m connected by cables to an electronic rack about the size of a filing cabinet, the entire station costing of the order of £80 000.

The high cost of these receiving stations has therefore excluded many potential users of the METE0SAT satellite data collection system (DCS). For this reason the European Space Agency (ESA) has devised the DCP/Retransmission system.

Data transmitted by DCPs are received at the ESA ground station at Darmstadt, West Germany, via the geostationary satellite METE0SAT 1 (Fig.l). A typical DCP climate station is shown in Fig.2.

FIG.l Data collection platforms at present transmit data at different times on specified channels to the satellite (e.g. METE0SAT, GOES, GMS etc.) for relay to the ESA ground receiving station where data are processed for distribution. If the platform user does not own a receiving station, data are received via the worldwide telex link known as the Global Telecommunication System (GTS).

The distribution of DCP messages to users at present is primarily using the GTS (Global Telecommunications System). Other means are also available such as telex and mail (for non-real-time data). All of these carriers are terrestial and require the use of existing means of communication. In many locations where DCP data could be used, these existing carriers are unreliable and expensive.

Collection of data using METEOSAT 111

Vf

/ ^ S

FIG.2 The automatic climate station uses an array of sensors to measure wind speed, wind direction, temperature, wet bulb depression, solar radiation, net radiation and rainfall. The data from these sensors are processed by the DCP microprocessor to compute potential evaporation using the Penman equation. The resulting data are transmitted by the DCP once per day.

THE DCP/WEFAX RETRANSMISSION SYSTEM

The European Space Agency transmits low resolution meteorological images of the earth via the Wefax link on METEOSAT 2 for reception on a secondary data user station (SDUS). This takes the form of a simple L band receiver linked to a facsimile machine. Image data are transmitted over a period of approximately 3i min with a 27 s pause between consecutive broadcasts. The new data retransmission service utilizes this 27 s "dead" period to retransmit DCP data. By this method a modified form of SDUS is used to receive retransmitted data which can be obtained within 2-6 min of the DCP transmission with an average of 3-4 min. During the 3j min of the

112 R.W.Herschy

image transmissions, all received DCP data from METEOSAT 1 at Darmstadt are stored in a buffer after being labelled with their time of acquisition. Upon termination of the image format, the DCP messages suitably packed, are transmitted until the buffer is empty or there is a request for a Wefax transmission. In the latter case, any remaining messages are held until the next transmission slot 4 min later. The new system is shown diagrammatically in Fig.3.

E SA GROUND STATION

FIG.3 The DCP/Retransmission system shown diagrammatic-ally . The DCP data are transmitted via METEOSAT 1 to ESA at the Darmstadt ground station. After processing at ESA the data are retransmitted to METEOSAT 2 for relay to the user's own ground station receiver. Two satellites are necessary since METEOSAT 2 does not have a DCP facility and METEOSAT 1 does not have a Wefax facility.

RECEIVER SYSTEM DESCRIPTION

The receiver is designed on a modular basis giving a variety of possible user options in display and storage and contains two major assemblies, the antenna unit and receiver unit the details of which are shown in Fig.4.

The antenna unit consists of a 1.5 m antenna and associated down converter. The latter amplifies the received signals, filters and down converts to VHF (1691 MHz to 133-139 MHz). Cross-site transmission to the receiver unit is carried out at this frequency. The receiver unit consists of the following sections (see Fig.4):

(a) The second down converter introduces a further conversion to an intermediate frequency of 10.7 MHz. This section also provides automatic frequency control and signal strength indication.

(b) The demodulator and bit conditioner recover the baseband signal and processes it to produce digital data and clock signals.

(c) The format decoder detects the presence of retransmitted data

Collection of data using METEOSAT 113

in the incoming bit stream, strips off all format coding and forwards the derived DCP data for storage and processing. The DCP data are also routed to a high speed interface.

(d) The storage and processing section performs the essential processing and storage required to interface the incoming DCP data with a variety of output options. Buffer storage is provided to ensure that, with the worst case data rate, the output options function without loss of data. Figures 5 and 6 show the antenna unit and receiver unit respectively. The complete system is offered at a price of about £15 000 but this is likely to be reduced depending on demand.

RECEIVER UNIT

DOWN CONVERTER - < E : 30-

2nd 00WN

CONVERTER

DEMOD­ULATOR

Ff BIT

H CONDI­TIONER

in FORMAT

0EC00ER 4=3>!N1 HIGH SPEED ERFACE

STORAGE

6.

PROCESSING

h P

—1 n3 1 3 !

PORT

RAUEL

NTRONCS

RS232

P O R T "

HIGH SPEED

PRINTER

SERIAL

PRINTER

TERMINAL

F LOPPY-DISK UNIT

(OPTIONAL)

FIG.4 Block diagram of the DCP/Retransmission system.

USER FACILITIES

Certain options are available in modular sub-units such as a mini­floppy disc unit which provides a mass storage medium for DCP data storage for up to 1500 DCP messages; a high speed printer which provides hard copy of received DCP data; and a user terminal which enables full control of the receiver facilities and interfacing via an RS 232 port. If Wefax imagery is required, this can be accomplished by the addition of standard display and processing equipment; weather images and DCP data can be viewed together.

The basic receiver unit includes the facility of a direct data link which transfers all acquired DCP data via the high speed inter­face. All data transferred by this method are unprocessed and hence identical to the original transmitted data. This provides the facility for processing by, for example, a host computer system. Control of the DCP receiver modes is achieved via a terminal

114 R.W.Herschy

FIG.5 The antenna unit.

connected to the RS 232 port. By this means stored DCP data can be routed to the printer and/or mini floppy disc storage. The terminal can also be used to create a look-up table in the receiver unit to enable only data from selected DCPs to be printed and/or stored. Additional header information, such as DCP names, can be entered by the user which is then printed or stored as part of the relevant data header. The use of this terminal is only required for setting-up and once operating in the required mode no further interaction is required.

MECHANICAL CONFIGURATION

The antenna and down converter assembly consists of a 1.5-m-dish antenna mounted on a 120 mm outer diameter pipe with the down converter enclosure below it. The location should be such that a "line of sight" is available to METEOSAT 2. The dish antenna may, however, be up to 100 m from the receiver unit.

Collection of data using METEOSAT 115

The receiver unit is housed in a small case which is located indoors in a position convenient to a power socket and to the printer or computer or other terminals. The front panel contains various status indicators for signal reception, channel selection switch for use with other geostationary satellites, alert indicator, DCP data indicator, stored data and Wefax data reception. The rear panel holds all the interface connectors for VHF input, high speed

• • « • • • n—y, ' '< • , Ç

FIG.6 The receiver unit.

interface, printer output, terminal interface, floppy disc unit, video output, power input, and alert message.

USER EXPERIENCE

Three DCP receivers are now operating in the UK. These are located at Nottingham, Reading and Bath serving the Severn Trent Water Authority, the Thames Water Authority and the Wessex Water Authority, respectively. These receivers form part of the DCP pilot investigat­ion already described and reported in Herschy (1982). The receivers were installed in late 198 3.

FUTURE AVAILABILITY OF ESA GEOSTATIONARY SATELLITES

Of crucial importance to present and potential users of satellite telemetry in a future operational system is the availability of

116 R.W.Herschy

satellites. At an intergovernmental conference in March 1983, European Meteorological Services agreed to participate in an Operational METEOSAT Programme having the following content and being executed by the European Space Agency:

METEOSAT 1 expected to serve until 1985 METEOSAT 2 expected to serve until 1986 METEOSAT Prototype 2 expected launch 1985 M0P1 expected launch 1987 M0P2 expected launch 1988 M0P3 expected launch 1990

The launch of the METEOSAT Prototype 2 is officially part of the pre­operational programme. Taking an optimistic view therefore it would seem that an uninterrupted DCS will be available up to the end of 1995. On the other hand, and taking the pessimistic view, satellites cannot be guaranteed a successful launch nor can it be guaranteed that all systems are operational even after a successful launch. It will be recalled that the DCP facility on METEOSAT 2 failed to operate after launch.

It would seem, therefore, that for an operational system the availability of a satellite must be assured. In addition a spare satellite as a back-up in the event of failure would also be necessary.

If these requirements are satisfied it would appear that satellite telemetry would become a real option and indeed a cost effective alternative to present-day terrestial systems where in the UK the allocation and availability of frequencies are becoming difficult. During the UK pilot investigation no charge was made by ESA for the use of the satellite and similar arrangements are offered to contributing member countries. No such arrangement has been agreed for a future operational system and indeed it would be expected that a charge, albeit perhaps a nominal one, would be made. Other possibilities could be studied including the users renting the satellite facility. All of these matters will no doubt be the subject of discussion and negotiation in the future.

CONCLUSIONS

The development by the European Space Agency of the DCP/ftetransmission system now offers the user a low cost method of satellite telemetry. The data can be received by the user within a period of 2-6 min of transmission and a variety of options are available for recording, displaying or archiving the data. The system will operate anywhere in the area covered by the ESA METEOSAT satellites. Provided the satellites are made available the system offers a cost effective alternative to terrestial telemetry.

ACKNOWLEDGEMENT Acknowledgement is made to the Department of the Environment, UK, for permission to publish this paper. Any views expressed in the paper, however, are those of the author and not necessarily those of the Department.

Collection of data using METEOSAT 117

REFERENCES

Herschy, R.W. (1982) Towards a satellite-based hydrometric data collection system. In: Advances in Hydrometry (Proc. Exeter Symp., July 1982), 285-296. IAHS Publ . no. 134.

Shope, W.G. & Paulson, R.W. (1980) Real time data collection via satellite for water management. ASCE Convention Exposition (Florida), October 80-561.